Remove test-support feature from codex-core and replace it with explicit test toggles

## Why

`codex-core` still had a `test-support` crate feature that was enabled by multiple
consumers (`core_test_support`, `app_test_support`, and `codex-tui` dev-deps).
That introduced a second feature-resolved shape of `codex-core`, which increases
compile cost and cache fragmentation across the workspace.

The same root issue previously affected `deterministic_process_ids`: we were
using crate features as a test/runtime switch, and Cargo had to build extra
variants of a very large crate.

## What Changed

### 1) Remove `test-support` as a crate feature

- Deleted the feature declaration from `core/Cargo.toml`.
- Removed `features = ["test-support"]` from:
  - `core/tests/common/Cargo.toml`
  - `app-server/tests/common/Cargo.toml`
  - `tui/Cargo.toml` (dev-dependency)
- Removed Bazel `crate_features = ["test-support"]` from `core/BUILD.bazel`.

### 2) Keep test behavior without feature-gated crate variants

- Converted test-only behavior toggles to **explicit runtime switches** backed
  by `AtomicBool`:
  - thread-manager test mode toggle
  - deterministic unified-exec process-id toggle
- Enabled those toggles from `core_test_support` ctor so integration tests keep
  deterministic and test-friendly behavior.

### 3) Replace feature-gated helper APIs with always-compiled hidden helpers

- APIs that were previously behind `#[cfg(feature = "test-support")]` are now
  available as `#[doc(hidden)]` test helpers, avoiding feature-split builds
  while preserving existing test call sites.

## Expected Benefits

### Concrete dependency-graph effect

`cargo tree -p codex-core -e features` now shows only the default feature path
for `codex-core`; there are no `test-support` or `deterministic_process_ids`
feature edges remaining.

### Build-performance impact

- Eliminates feature-driven duplicate `codex-core` compilation variants.
- Improves cache reuse across test-support consumers that previously forced
  separate feature resolution.
- Reduces rebuild churn when switching between targets that did and did not
  depend on the feature-enabled `codex-core` shape.

## Safety / behavior notes

- Production behavior remains unchanged by default.
- Test-only behavior is now explicit and opt-in via dedicated test toggles,
  with docstrings clarifying these must stay at default values in production.

## Validation

- `just fmt`
- `cargo test -p codex-core unified_exec::`
- `cargo test -p codex-core --test all unified_exec -- --test-threads=1`
- `cargo check -p app_test_support`
- `cargo check -p codex-tui --tests`
- `cargo check -p codex-core`
- `cargo tree -p codex-core -e features`

codex-rs/app-server/tests/common/Cargo.toml

@@ -12,7 +12,7 @@ anyhow = { workspace = true }
 base64 = { workspace = true }
 chrono = { workspace = true }
 codex-app-server-protocol = { workspace = true }
-codex-core = { workspace = true, features = ["test-support"] }
+codex-core = { workspace = true }
 codex-protocol = { workspace = true }
 codex-utils-cargo-bin = { workspace = true }
 serde = { workspace = true }